Lecture 10 Lignin Biosynthesis II Wood Chemistry PSE 406 Lecture 10 Lignin Biosynthesis II 1

Class Agenda Possible linkages Free radical coupling example Isolation of lignin Linkages b-O-4, 4-0-5, 5-5, b-b, b-5, b-1 Appendix (coupling mechanisms)

Lignin-an example

Possible Lignin Linkages If you look at this precursor, you can see (ignore the 3 position) that there are several possible linkage combinations. Not all of these occur. Possible Combinations b 1 4-O 5 b-1 1-0-4 4-O-5 5-5 b-O-4 1-5 4-O-O-4 b-5 1-1 b-b

Common Lignin Linkages The linkages shown on the right are those formed in dehydrogenation polymers and also found in wood. All but the a-O-4 linkage are formed through free radical coupling.

How Do We Know What Linkages are Found in Actual Lignin? In order to study lignin, we need to isolate it. Isolation procedures modify lignin structure. It is not possible to isolate all of the lignin. As a polymer, it is difficult to determine all of the structural elements Analysis of lignin after isolation: Use of polymer analytical techniques (H1NMR, C13NMR, IR, UV,etc.) Break the lignin into fragments, analyze the fragments, and put the puzzle back together.

Lignin Isolation Procedures Insoluble Lignin Preparations: Klason, Periodate, etc Destruction of carbohydrates with acids, enzymes, etc: Lignin structure highly altered. Soluble Lignin Preparations: Bjorkman (MWL), Brauns, etc Lignin is removed from wood (solublized) using different solvents and mechanical techniques. These materials are considered most representative of native lignin (MWL #1). Very low yields. Neutral Solvents (Example: MWL, ball milling ® dioxane extraction Acidic Organic Solvents (Example: Alcohol lignin) Commercial Lignin Preparations Highly degraded materials including Kraft lignin, lignosulfonates, hydrolysis lignin, etc.

Lignin Linkages: b-O-4 Easily the most prevalent linkage. Roughly 50% of softwood lignin linkages. As high as 60% in hardwood lignin. Dimer structure name: Aryl glycerol b-aryl ether Reactive linkage in alkaline pulping systems

Lignin Linkages: b-1 Minor linkage: estimates range between <2 to 7% in softwood lignin. Dimer structure name: 1,2-Diaryl propane Formation of this linkage requires a splitting off of one of the side chains (see next slide).

Displacement of Side Chain in b-1 Formation The side chain which is split off in this coupling reaction if it is attached b-O-4 as in this example is known as: glyceraldehyde-b-aryl ether. The amount of this structure in softwood has been estimated at 2%.

Lignin Linkages: b-5 7-12% of softwood linkages ~5% of hardwood linkages The cyclic type of b-5 linked structure shown on the right is known as: Phenylcoumaran The are a small amount of non-cyclic b-5 structures. Notice that this dimer also contains a a-O-4 linkage

Lignin Linkages: b-b Very minor component in hardwoods and softwoods (2-3%) Dimer structure name: Pinoresinol The structure on the right is a wood extractive

Lignin Linkages: 4-0-5 Linkage makes up roughly 4% of softwood and 7% of hardwood linkages. Dimer structure name: Diaryl ether Even though this is an ether linkage, it is not cleaved under pulping conditions.

Lignin Linkages: 5-5 The amount of 5-5 linkages in lignin is debated. ~ 10-13 % in softwoods ~5% in hardwoods Dimer structure name: Biphenyl Even though this is an ether linkage, it is not cleaved under pulping conditions.

Lignin Linkages: a-O-4 There are 3 possibilities for a-O-4 linkages Phenylcoumarin structures (b-5) Free a-O-4 (Structure A) Eight member ring NMR work by Brunow indicates that there are essentially no free a-O-4. All of these linkages are linked to biphenyl structures (5-5). (Structure B)

Summary of Lignin Linkage Frequencies

Lecture 10 Reactions

Lignin Coupling 4-O-5

Lignin Coupling -5

Lignin Coupling b-b

Lignin Coupling b-1

Lignin Coupling 5-5